Acoustic device



May 12, 1931. O N 1,804,689

ACOUSTIC DEVICE Filed April 30, 1926 A W074 //a/m ,6 it! Zia.

I Patented May 12, 1931 UNITED STATES PATENT OFFICE n. nomnmn, or nonrcmm, mew masmr. assxenoa '10 ELEO- 'rmc comm, or NEW YORK, n. Y., A coaroaa'nox or new roux acousrrc amon- Application filed April 80,

This invention relates .to acoustic elements and more particularly to such elements, the vibration of which may cause unpleasant effects on reproduced sound.

An-object of this invention is the elimination or reduction of undesired vibrations in acoustic devices. I

Another object of the invention is toprevent distortion due to the introduction of extraneous or parasitic vibrations introduced by the elements of aphonograph system.

Certain parts of prior phonographic devices notably the tone arms are prone to vibrate as a result of the vibrations impressed directly thereon through the air column and .indirectly thereon through the framecarrying the needle system and these vibrations are reflected both directly on the vibrations in the air column and indirectly through the needle system thereby distorting the sounds being reproduced.

In accordance with the present invention the vibration of various elements at sound wave frequencies is substantially reduced or prevented b making them of sections, preferably longitudinal, which transmit sound vibrations at different velocities. These sections are not Welded, soldered or otherwise rigidly secured together to, form an integral unit, but are merely held together in. intimate frictional contact throughout their lengths so that each section, in effect, remains a unit. Since the velocity of transmission of sound in one section is different from that in the adjoining section or sections, transmitted vibrations will be out of phase and the resulting friction between the sections will quicklydissipate the energy of such vibrations. Specific forms of the invention are covered in the following description taken in connection with the attached drawings in which Fig. 1 is a top view of a phonograph tone arm, Fig. 2 is a side elevation in partial section of the same, Fig. 3 illustrates an alternative method whereby an acoustic element may be made of different shaped parts of the same material, and Fig. 4 is a view, partly in section, of an electromagnetic unit to which 60 the invention has been applied.

1926. Serial 1T0. 105,747.

phragm 12 WhlCh sets up air vibrations whlch in turn are transmitted through the tone arm 13 to a horn not-shown. The tone arm 13 is constructed in two parts, 14 and 15 composed of different metals or materials which transmit sound vibrations at different velocities. The contacting surfaces which extend the whole length of these parts are made as smooth as possible to provide intimate contact between them; In use and to form the tone arm they are clamped together by bands 16 and 17, or similar means.

As stated above, the two parts are not welded, soldered or otherwise rigidly secured together, but are held in intimate contact with each other substantially throughout their lengths so that any movements of the sections which are out of phase with each other, will cause friction between the sections and consequently a dissipation of the vibratory energy. In tone arms composed of a single material or having a unitary construction, vibrations from the diaphragm are transmitted to the tone arm through both air and metallic paths, and the tone arm vibrations are impressed on the air column enclosed therein and cause distortion. In the structure of this invention, the velocities of propagation of wave energy at sound frequencies in the two media are different and hence vibrations therethrough will be out of phase causing slippage between the surfaces in contact and resulting in a loss in wave energy due to the friction therebetween. It will therefore be seen that disgreater the difference in velocity the greater the slippage and consequently the greater the damping.

Fig. 3 is a longitudinal section through a part of an acoustic element, such as a tone arm, and shows an alternative arrangement of the parts. The surface of section. 24 is uniform while the surface of section 25 is warped or corrugated. Vibrations through these two parts will be out of phase since the path in the corrugated part 25 is longer than a that in the-uniform part 24. Since the vibrations are out of phase and the two sections are held closely in contact, there will be friction between the'parts resulting in a mutual damping effect.

Fig. 4 shows in partial section the invention as applied tothe armature of an electromag netic reproducing and recording unit. The armature, to which the stylus 18 is attached, is composed of a main body 19 of iron or other magnetic substance having plates 20 of copper or other metal with a velocity of transmission different from iron, fastened on either sideof the iron armature by means of a screw 21 or an equivalent. It is of course understood that the windings 22 surrounding the armature, together with the armature, are included in a magnetic circuit set up by the permanent magnet 23.

In the embodiment of Fig. 4; the most noticeable effect will occur at the resonant period. When the velocity of transmission, the density or mass and elasticity of the' transmittin medium are such that vibrations of a given requency, set up in one portion of the medium, are in phase with the vibrations re fiected as they go from one medium to another, the medium is said to be in resonance at that frequency. If, however, the vibrations in the medium are damped out before they are reflected, there will be no resonance. Having materials in which the sound vibrations travel at different velocities closely fixed to each other, the vibrations will be out of phase and hence the friction between such materials will suppress the vibrations.

In the tone arms of Figs. 1 and 2 and Fig. 3,

the matter of resonance is alsoimportant but since the Vibrations are suppressed resonance will not occur.

What is claimed is:

1. An acoustic element, comprising two adjacent and co-extensive similar longitudinal sections in friction relation with each other, said sections having different sound velocity coeflicients.

2. An acoustic element, comprising a plurality of longitudinal and co-extensive sections of resonant metallic materials which transmit sound vibrations at different velocities, means for holding said sections in longitudinal contact with each other, so that any motion of one section which is not common to all is opposed by the friction between this one section and the other sections with which it is in contact.

3. A phonograph tone arm, comprising two longitudinally adjacent sections of resonant metallic materials in intimate contact, between their ends, each section having a different sound velocity coefficient and forming the stills support for the other throughout its lengt l 4. A phonograph tone arm, comprising two longitudinal and coextensive sections of resonant metallic materials in intimate contact between their ends, one of said sections being steel, another of said sections being brass.

5. An acoustic element, comprising a plurality of longitudinal and (to-extensive members of resonant metallic materials in intimate contact at a s s of points and presenting to sound vibrations, paths of different lengths whereby such vibrations will be out of phase at contacting points.

6. An acoustic element, comprising a plurality of longitudinal and co-extensive members of resonant metallic materials, one of said members being uniform, and another of said members being corrugated the ridges thereof being in intimate contact with said uniform members.

7. A support for phonograph reproducers and recorders comprisingtwo hollow metal members, and means for securing said mem bers in intimate longitudinal contact to form a conduit, each of said members having a different sound velocity coefficient.

In witness whereof, I hereunto subscribe my name this 29 day of April A. D., 1926.

NEWMAN H. HOLLAND. 

